The present invention relates to a continuous drying system for drying a flake-formed or granular solid, or a small piece of solid, and particularly relates to a continuous drying system to dry effectively sliced vegetables such as a cabbage, a carrot, a leek, an onion, burdock, and garlic or sliced fruits such as an apple, a persimmon, a banana, etc. or small pieces of tea leaves and medicinal herbs or granular foods such as rice, wheat and barley, beans, etc., without deterioration of their quality.
Conventionally, various continuous drying systems have been proposed by using a horizontal drum mixer, as drying means for drying effectively by transfer of powder and granular bodies stirring. For example, FIG. 11(a) and FIG. 11(b) are sectional block diagrams illustrating a screw mixer and a paddle mixer, respectively, as representative conventional examples of drum mixers.
The screw mixer has a spiral ribbon screw 32 engaged with the axis of rotation 31 supported on the shaft center of a horizontal cylindrical container 30 and a solid body is transferred in to one direction stirring by the ribbon screw. On the other hand, the paddle mixer has many plate stirring wings 33--replaced to the ribbon screw--tilted in a certain direction against the axis of rotation 31 engaged with the end of an arm.
In any systems aforementioned, drying is carried out by stirring during transferring a drying object from one end to the other end of a container 30 with a screw 32 or a paddle 33 on the basis of passing heated air through the cylindrical container 30 (a drum) or heating the cylindrical container 30 from outside.
However, in the continuous drying system by using the screw mixer and the paddle mixer, the increment of the rotation speed of the axis of rotation for reinforcing stirring increases a transfer speed of the drying object proportionally. Therefore, it is difficult to control freely the time for retention and drying of the drying object in the cylindrical container contacting enough with a heated gas by vigorous stirring.
There are some problems: differences in the shape, size, water content, and drying rate of the drying object; necessities of regulating the drying object to supply to the drying system to an optimal quantity. In conventional drying systems, rotation of the screw 32 or the paddle 33 strongly relates to transfer speed to make independent regulation difficult. Thus, it is required to design the drying system according to the properties of the drying object. Subsequently, drying process by using a single system is difficult for different drying objects.
On the other hand, in order to yield a dried food giving a good flavor and taste, drying is necessary at an appropriate temperature to prevent a considerable change of the quality of a material. A common method for yielding a dried food giving a good flavor and taste is drying in the sun and a natural wind for preparation of a dried fish. However, this method is inappropriate for industrial mass production due to the effect of natural environment such as weather and seasons, a place for drying is restricted, and a low productivity caused by a large consumption of labor and time for drying function.
Industrial drying methods so far frequently employed include a method for drying by contacting gas discharged by burning of light oil directly with a drying object or a method by contacting the drying object with a high temperature air generated by a heat exchanger.
However, in consideration of the conditions of natural drying to yield a dried fish having satisfactory flavor and taste, mild drying at a normal temperature or a temperature near a room temperature is preferable as possible. However, this condition is difficult to realize industrially. There are problems, for example: when the temperature of a food under drying becomes relatively high such as 50.degree. C. its activate performance of enzymes in a food to oxidize and easily decompose a useful component and flavor component finally resulting in deterioration of taste. In addition, a high drying temperature causes unevenly dried and excessively dried parts, denaturalization of the food, and deterioration of eating sensation (chewing sensation).
Drying rate decreases greatly at a low drying temperature near a room temperature such as 40.degree. C. or under. Therefore, drying at these temperatures requires that the drying object be contacted with environmental air as possible, practically by vigorous stirring.
Drying vegetables, fruits, fishes, etc. requires to adjust water remained in a food after drying to appropriate content. Excessive drying causes deterioration of a flavor of the food and hardens the food, and, on the contrary, insufficient drying keeps the food soft, but does not allow long term preservation easily. Therefore, it is required to adjust remained water content in a given narrow range. For the purpose, it is important to regulate drying time accurately.
As described before, there is a problem where it is difficult to realize the following double operations in the conventional drum mixer: regulating freely the transfer speed of a solid body accompanied with stirring satisfactorily the same.
Thus, the present invention is intended to overcome the problems in the prior art by providing a drying means having large drying rate by improving contact of a drying object with an ambient gas through a stirring means capable of increasing the degree of dryness without increasing the transfer speed of the drying object in continuous drying of a flake-formed or granular solid.
In addition, the present invention provides a means for regulating retention time, i.e. drying time, of the drying object in a drying room to a given value by free regulation of the transfer speed of the drying object even under a very vigorous stirring condition.
Further, the present invention provides a means for yielding preservative equipment and flavorful dried foods or dried medicinal herbs by drying effectively and evenly flake-formed or granular foods at a low drying temperature in combination with the aforementioned stirring means with an appropriate heating means.